FanFiction.Net is the self-proclaimed "world's largest fanfiction archive and forum" and is one of if not the largest site hosting fanfiction. Has a sister site Fictionpress, much smaller, identical layout.

It is in the process of being preemptively archived.

Current status

Profile IDs are being scraped from indices; downloading will then occur on a per-profile basis. Drop by the IRC channel (#fanfriction EFNet) for more information.